An interference filter, in which a dielectric thin film is formed on a glass substrate, transmits light of a specific wavelength band and reflects light of other wavelength bands. Such an interference filter is, for example, incorporated into an optical component constituting an optical communication system and used for separating light into light of wavelengths required for optical communication and light of other wavelengths (such as noise). FIGS. 1A and 1B show the structure of an optical component 1 including an interference filter 3. FIG. 1A is a cross-sectional view of the optical component 1 taken along a plane including an optical axis 5 and FIG. 1B is an enlarged view of the circle shown in FIG. 1A. In this optical component 1, two optical fibers (41, 42), which are retained by respective ferrules (61, 62), are arranged on the optical axis 5 in such a manner that opening ends (43, 44) face with each other. An optical path is formed between these opposing opening ends (43, 44). Further, the optical component 1 has a configuration in which light Lin (incident light Lin) entering the optical fiber 41 on one side is emitted as output light Lout from the optical fiber 42 on the other side.
Where a direction in which the optical axis 5 extends is defined as the front-back direction, the interference filter 3 is arranged near the center between the opening ends (43, 44) of the front and back optical fibers (41, 42) and collimator lenses (51, 52) are respectively arranged before and after the interference filter 3. Further, in the illustrated optical component 1, collimators (33, 34) are each connected to the front and back of a housing 2 which accommodates the interference filter 3, the collimators (33, 34) respectively including housings (31, 32) which respectively accommodate the ferrules (61, 52) retaining the optical fibers (41, 42) and the collimator lenses (51, 52).
The operation of the optical component 1 will be described below referring to FIGS. 1A and 1B. In these figures, in order to be able to easily distinguish the forward path and the return path, only one of these optical paths that are symmetrical about the optical axis 5 is shown; the forward path is an optical path directed from the input-side optical fiber 41 toward the interference filter 3 and the return path is an optical path directed from the interference filter 3 toward the input-side optical fiber 41. In the optical component 1, the input light Lin from a light source first enters the input-side (front) optical fiber 41. The input light Lin is emitted from the opening end (back end) 43 of the same optical fiber 41, and this emitted light subsequently enters the interference filter 3, as parallel beams L1, through the front collimator lens 51. Only part of the incoming light L1 that has a specific wavelength band is transmitted backward through the interference filter 3. The transmitted light L2 couples to the opening end (front end) 44 of the output-side (back) optical fiber 42 through the back collimator lens 52. Meanwhile, light L3 which is not transmitted through the interference filter 3 is reflected. It should be noted that the illustrated optical component 1 has a straight optical path extending from the input side to the output side of light. So, in order to prevent the light L3 reflected by the interference filter 3 from directly entering to the input-side collimator lens 51 and coupling to the input-side optical fiber 41, the tilt angle θ between the optical axis 5 and the normal line 4 of the light incident plane of the interference filter 3 is about 2° to 5°. Thereby, the light L3 reflected by the interference filter 3 is absorbed by the inner surface of the housing 2.
Incidentally, the light L3 reflected by the interference filter 3 is directed to the inner surface of the housing 2; however, this reflected light L3 is not completely absorbed by the inner surface of the housing 2 and a part thereof is reflected by the inner surface of the housing 2. In addition, since the inner surface of the housing 2 is not a specular surface, the light L3 reflected by the interference filter 3 is not specularly reflected by the inner surface of the housing 2 and is scattered as illustrated. Consequently, the scattered lights L4 are further repeatedly reflected and scattered by the inner surface of the housing 2; that is, so-called “stray light” is generated. The stray light eventually disappears because most of the energy of the stray light is absorbed and converted to heat when the stray light is scattered at the inner surface of the housing 2. However, light L5, which is a part of the light L4 generated when the light L3 reflected by the interference filter 3 is initially scattered at the inner surface housing 2, may travel back exactly the same optical path as that of the light L3 reflected by the interference filter 3. In this case, the light L5 of the scattered lights L4 returns the path to the interference filter 3 traversed by the emitted light L1 from the input-side optical fiber 41. Thus, the light L5 becomes “back reflected light” L6 which re-couples to the input-side optical fiber 41.
There is a possibility that this back reflected light L6 then propagates toward the light source through the input-side optical fiber 41 and reach the light source. In optical communication, a semiconductor laser element is used as a light source. Such optical feedback into the light-emitting section of a semiconductor laser causes the change of the oscillation characteristics of the semiconductor laser. This makes the output of the laser unstable, which results in deterioration in the quality of optical communication. An index of the degree of this deterioration is known as “return loss” and a method of measuring it is described in Non-Patent Literature 1 listed below and the like.
The Patent Literatures 1 to 3 listed below describe countermeasures against stray light in optical components. The Patent Literature 1 describes the invention using the following countermeasure to prevent a part of light reflected by an interference filter from entering a light-receiving element arranged on the light output-side, which causes crosstalk. That is, the housing of the invention has a conical hole whose opening size is larger than the spread of the light reflected by the interference filter and gradually reduced in the depth direction; this allows the inner surface of the conical hole to reflect the reflected light and to prevent the reflected light from traveling on the optical path to the light-receiving element.
Patent Literatures 2 and 3 describe optical components in which a material for absorbing stray light is used in the inner surface of the housing.